Populations

Life History Traits

Life history traits include significant life events of a species, such as reproductive rate and lifespan.
A life history trait describes the timing and characteristics of a significant event in the life cycle of a species. Life history traits determine population reproductive rates. Life history traits of a species include the growth rate, age at first reproduction, number and size of offspring, reproductive lifespan, and patterns of aging.

These traits are directly related to natural selection—the concept that individuals best adapted for survival will be those that reproduce the most and pass on their traits. In natural selection, some traits may grant individuals an advantage by surviving to reproductive age or producing offspring that also survive. There will thus be more individuals with those traits than with traits that do not confer any advantage, or even confer a disadvantage. Thus, the traits that confer an advantage will become more common than ones that do not.

Life History Traits of Three Species

Life History Trait Mouse Bobcat Elephant
Age at first reproduction 6–8 weeks 1–2 years 10–12 years
Average number of offspring per birth 6–8 2–4 1–2
Average lifespan 2 years 12 years 60–70 years

Life history traits include such factors as age at first reproduction, number of offspring, and lifespan. A comparison of life history traits for several species shows how these traits can vary.

Growth rate describes how quickly individuals of a species reach maturity. Factors affecting growth rate include foraging and hunting strategies, as well as strategies that provide protection. For example, some animal species engage in signaling behaviors to alert members of their community that predators are approaching or that enemies are near, such as prairie dogs who make specific vocalizations to alert members of their community when predators are nearby.

Many life histories consider only the reproductive history of a species: the age at which reproduction may begin, the frequency of reproduction, and the rate at which offspring are born. Among plants, for example, bamboo produces seeds once during its lifespan, then the plant dies. This is also true of many grains. Among animals, butterflies and many spiders mate, lay eggs, then die. Other species of plants and animals reproduce repeatedly and with different levels of frequency. Humans, cows, and giraffes mate and reproduce, yielding usually one offspring at a time. Oak trees, apple trees, and hibiscus plants produce flowers and seeds, continuing to reproduce every year. An oak tree might produce several thousand acorns, but only a handful might sprout to grow into saplings.

Patterns of aging can have significant influences on the other traits. Although members of many species die upon completion of successful mating, others live much longer lives, ensuring their offspring receive the care they require to reach reproductive age themselves. For example, elephant herds often consist of many generations, the oldest ones nurturing and protecting the younger ones, ensuring genetic lines are maintained.

Life and Cohort Tables

A life table shows the developmental stages of a single reproductive event of a species. It can be generated using a cohort table, which follows many individuals of a species throughout their lives.
Life history traits affect the life table. A life table is a record of the developmental stages of one reproductive event of a species. The survival rate for members in a population of insects can be displayed in a cohort table. For example, a female insect lays 200 eggs. Birds devour 100 of the eggs, indicating a mortality rate of 50% at the egg-development stage. The larvae feed on leaves, but a storm has torn a high percentage of leaves from trees, leaving the larvae with minimal food, and 80 die. The remaining 20 larvae develop into pupae, but a late snow storm freezes 15 of 20 pupae, or 75%. Only 5 of the original 200 insects in the group survive to adulthood.

A cohort table illustrates a cohort of organisms that are followed through their life stages. It is used to generate a life table, which describes the likelihood (probability) of surviving from one life stage to another.

Life and Mortality Rate Table

Developmental Stage Living Cause of Mortality Number Deceased Mortality Rate
Egg 200 Predation 100 50%
Larva 100 Loss of food supply 80 80%
Pupa 20 Climate 15 75%
Adult 5

Life and mortality tables show the percentage of individuals who die at a given time. This is useful when determining changes in population sizes.

Survivorship Curves

Survivorship curves depend on mortality rates and age structure.
A survivorship curve is a plot of the proportion of individuals alive at each stage of their life cycle or at a certain age. Survivorship curves are categorized into three types: type I, type II, or type III. Type I curves indicate high survival rates for populations of infants with a decrease in survival at old age. Humans, rhinos, and elephants fall under the type I curve category. Type II curves show fairly regular or constant death rates from birth through old age. Examples of species that are categorized under type II curves are birds and squirrels, which have fairly even mortality rates from the egg (for birds) or kit (for squirrels) stage through to adulthood. Type III curves show high mortality rates of most offspring at early stages, and then high survival rates once the individuals reach adulthood. Examples of type III species are fish and green sea turtles, which lay large numbers of eggs and spend little or no time parenting, which results in few offspring that reach adulthood. A graph of the three survivorship curves shows this difference. Remember that a reduced lifespan may be the result of predation, disease, accident, loss of food supply, or severe weather patterns at any time in an individual's life. Type I: Rhinos have low initial death rates from birth through to middle age. Their survivorship curve remains fairly level through youth and middle age, followed by a sharp downtrend when rhinos reach old age, usually between 40 and 50 years. Type II: The death rates of robins are fairly steady from egg to fledgling and to the end of the species' normal lifespan of about two years. Type III: As with many freshwater fish species, rainbow trout lay and fertilize thousands of eggs. The loss rate for fry is initially quite high, due primarily to predation by other fish, birds, bears, and otters. After the initial loss of so many individuals, the survivorship curve of rainbow trout flattens out toward the end of their lifespan, at roughly four to six years.
A survivorship curve is a tool ecologists use to show the proportion of individuals surviving through each stage of life. The type I survivorship curve shows minimal death rates at early ages. Humans, rhinos, and elephants fall under the type I curve category. The type II survivorship curve shows constant depletion of populations, such as the constant loss rate of a robin. The type III survivorship curve shows high initial population loss that subsequently flattens out for survivors, as exhibited in many freshwater fish species.

Fecundity and Mortality

Populations increase when the ability to reproduce (fecundity) exceeds the loss of individuals (mortality).
Reproduction increases population size, and death decreases population levels. Fecundity is the rate at which an individual of a species can produce offspring. Mortality is the rate of death in a population. Both fecundity and mortality rates change as a population reaches carrying capacity within an ecosystem. Carrying capacity is defined as the maximum number of individuals in a population that a habitat can support.

Fecundity varies by species. The ability to procreate, and the rate at which a species reproduces, influences the population. Fecundity can be measured in several ways: by the number of births in a population over a certain period of time, by the number of births per female over a given time period, or by births per thousand individuals in a population over a specific time period. Fecundity rates can be high or low. In species that produce frequent litters or clutches, fecundity is high. For example, a female house mouse can begin to mate at 4 to 7 weeks of age and produce a litter every 3 weeks after that. A litter is usually 5 or 6 mice, and that same female can deliver 5 to 10 litters per year. Thus, the fecundity of mice is high. On the other hand, some species reproduce infrequently and have only one offspring at a time, so their fecundity is low. For example, a female elephant does not reach maturity until 10 to 12 years old. A female elephant is pregnant for over eighteen months and generally delivers one or two infants. Elephants reproduce at a rate of one offspring every four years; thus, the fecundity rate of elephants is low. Environmental stresses, such as low food supply, influence fecundity rates for all species.

Mortality is considered a natural balance to reproduction. Mortality may be the result of disease, low food supply, extreme weather, predation, age, or other similar factors. The mortality rate of a species may be given as percentages of mortality by age group. For example, the mortality of elephants is 40% for newborns, which means that if a female elephant gives birth to six offspring during her lifetime, only two or three will likely survive beyond infancy. Loggerhead sea turtles, on the other hand, may lay a clutch of more than 100 eggs, but the predation of eggs and hatchlings is high. Of 1,000 eggs, only one offspring may actually survive to grow to reproductive adulthood at 15 to 25 years old.
The more offspring a species produces over a given period of time, the higher the fecundity rate (the rate at which a population can produce young). For example, a female house mouse can begin to mate at four to seven weeks of age and produce one litter every three weeks after that. Thus, the fecundity of mice is considered to be high. A female elephant, on the other hand, does not reach maturity until thirteen years old, is pregnant for eighteen months, and generally delivers one infant. Elephants reproduce at a rate of one offspring every four years; thus, the fecundity rate of elephants is considered to be low. Birds have a modest fecundity rate. The clutch size, or number of eggs produced at one time, in robins is three to five.